[Conker]

Note that I'm not (yet) a mechanical engineer or whatever, but reading this is what I'm thinking:

1- While working for the "air cushion" idea, wouldn't having an empty head cone cause a flight stability issue?

2- Do you really think that this "over pressure" created will be powerful enough and -especially- happen fast enough to actually disperse the BBs to be effective?

3- While improving flight stability, having rotation fins will cause drag, thus reducing its practical range.

Also, probably that to have enought pressure to get the "warhead" to go far, it would be best to use CO2 shells. Also, a little suggestion: as AI has done with their Tornado grenades, these are bright colours makes it easier to find them. And you might know that AI are actually working on their own project similar to that one. It was shown in the news section of this website, not that long ago.